This invention relates generally to surface acoustic wave devices and more particularly to surface acoustic wave device packages.
As is known in the art, surface acoustic wave devices (SAW devices) are used in a variety of applications such as resonators and delay lines for oscillator circuits, as well as, filters and pressure transducers. Generally, a SAW device comprises at least one transducer including a set of conductive members which is disposed on, or recessed within the upper surface of, a piezoelectric substrate. In many applications for SAW devices, particularly with respect to applications involving oscillators, it is important to provide a package having a relatively small size, while at the same time, properly mounting the SAW device within the package to reduce the so-called vibration sensitivity of the SAW device. It is known that the resonant frequency of an oscillator including a SAW device is sensitive to external vibration or the changes in external stresses applied to the SAW device. This sensitivity results from the external stress on the piezoelectric substrate causing changes in surface wave velocity and hence resonant frequency. Conventional packages, such as TO-8 packages and flat packs while being hermetically sealed, are relatively large in comparison to the size of the SAW device. Such packages also constrain how the SAW device substrate can be mounted within the package to provide a SAW device having a surface wave velocity which is relatively invariant with external stress.
It is generally known that the lowest vibration sensitivity is obtained when the bottom of the SAW device substrate is uniformly supported. One approach is to provide a soft, stress damping material such as a rubber or room temperature vulcanizing silicone rubber (RTV) on the conventional package to uniformly support the bottom of the SAW device substrate. It has been found, however, that such a soft material causes long term frequency shifts in SAW devices provided within such packages because over time the soft material will out-gas impurities which may become deposited upon the upper surface of the SAW device substrate. These deposited impurities are believed to cause changes in the velocity of the surface waves which propagate along the upper surface of the substrate and hence changes in the long term frequency characteristics of the device. These frequency characteristic changes often are of a magnitude equal to or greater than the changes caused by the very vibration effects that the soft material was to suppress. On the other hand, rigidly fastening the bottom of the substrate to the package may also be unacceptable since the thermal expansion characteristics of the SAW substrate are generally not perfectly matched to the thermal expansion characteristics of the material of the package. Because of the mismatch in thermal expansion characteristics, this arrangement leads to unpredictable temperature dependent stress characteristics that adversely affect frequency stability and may even result in fracture of the package.
One solution to this problem has been to provide a hermetically sealed package in which the SAW substrate provides a portion of the package. Such arrangements are shown in U.S. Pat. No. 4,270,105, Parker et al, issued May 26, 1981, entitled "Stabilized Surface Wave Device," and assigned to the same assignee as the present invention, and articles entitled "Long Term Aging and Mechanical Stability of 1.4 GHz SAW Oscillators," by M. Gilden et al, Proceedings of the IEEE, Ultrasonic Symposium, 1980, pp. 184; and "SAW Resonator Frit Bonded Pressure Transducer," by D. Weirauch et al, Proceedings of the IEEE, Ultrasonic Symposium, 1979, pp. 874. The issued U.S. patent describes an all quartz package having a hydrophonic polymer coating for passivating the upper surface of the substrate on which the surface waves propagate. The article by Gilden et al describes a SAW based oscillator including a quartz packaged SAW device. Long term aging data indicates that such devices will have a frequency shift of at least .+-.4 ppm per year. For some applications of stable oscillators, this drift or aging characteristic is unacceptable. The second article by Weirauch describes a pressure transducer fabricated having an all-quartz package. According to this article, a frit was applied to both substrates and the substrates were then mated together. As indicated in this article, a time dependent short term change causing a hysteresis effect was present. This indicates that there may be some noise problem associated with this technique. It was also suggested that some long-term effects may be present. These short term and potential long term drift problems may be unacceptable for SAW devices such as delay lines and resonators when used in highly stable precision oscillators.